Oh DAYUM, 3 years?, I suppose i can give some, Either talk to her about it (Decent choice but i know she wont listen) Tell her parents about what she posts and does to your friends (Probably the best since this makes the parents curious about their child) - Added part to this, Tell her parents to check her phone for it. And for 3, Try to conduct a final plan on what to do when you are planning to talk to her, But the main topic should be as of why and what are the reasons she does this and does she even feel remorse or any emotion about a persons emotions, And ask her, (What will you do if you were me and i were you?, Wouldnt you feel exhausted and tired of life?, Telling lies about me and sexually harassing my friend, Even drawing porn about my friend, What gratification is it that you so seek from us?, Tell me.)
Now you may combine all these 3 to be a singularity of a solution, It may help!, But i give you this idea.